Radioresistance of cats has been seen in animal radiotherapy. Feline radioresistance and its underlying mechanism(s) were investigated in fibroblast cells and lymphocytes. We hypothesized that radioresistance was attributable to an increase in the cells ability to repair DNA damage. To investigate this hypothesis, fibroblast cells were exposed to various doses of X rays and then colony formation assays were performed. Survival curves showed that potential lethal damage repair (PLDR) for feline cells were greater than that for human cells. γ-H2AX foci assays were performed to evaluate DNA double-strand breaks (DSBs) formation and repair kinetics. After PLDR, feline cells displayed a decreased residual amount of γ-H2AX foci. Formation of chromosome aberrations (dicentrics) after PLDR as an indicator of radiation-induced DNA damage and repair; human, feline and canine lymphocytes were evaluated. Human and canine lymphocytes showed two to three times the number of dicentrics compared to feline lymphocytes. Finally, micronuclei assays were performed to further confirm the radioresistant nature of feline lymphocytes. In concordance with the results of the chromosome aberration assay, the number of micronuclei in feline lymphocytes was less than observed in human and canine lymphocytes. Taken together, these results show that DNA and chromosome damage induced by X irradiation is more effectively repaired in feline cells, resulting in less residual damage. Our results suggest that both feline fibroblasts and lymphocytes are more radioresistant compared to human cells of similar tissues, and this resistance can be contributed, at least in part, to greater ability for PLDR.